Every recorder quote comes down to one question from the owner: how many days of video will it keep? Working out how much storage for security cameras a job needs is simple arithmetic once you know each camera’s bitrate, how long it records each day and how many days the owner wants to keep. This guide shows the formula, works one example with every assumption stated, and covers the settings that move the answer.
How much storage do security cameras need?
Multiply each camera’s bitrate in megabits per second by the seconds it records each day, multiply by the number of cameras and the retention days, then divide by 8 to get megabytes. Eight cameras recording 4 Mbps around the clock for 30 days need about 10.4 TB, before headroom.
How much storage for security cameras: the formula
Cameras and recorders describe their streams in megabits per second (Mbps). Drives are sold in bytes, and there are 8 bits in a byte. So the whole calculation is:
Storage (MB) = bitrate (Mbps) x seconds recorded x cameras / 8
Divide by 1,000 for gigabytes and again for terabytes, the decimal units drive makers use. One handy figure: a stream of 1 Mbps recorded for a full day (86,400 seconds) fills 10.8 GB. Multiply that by the bitrate, the camera count and the retention days and you have the raw capacity.
This is what a security camera storage calculator does behind its sliders. Doing it by hand shows which input matters most on a given job, and it is usually the bitrate.
A worked example with stated assumptions
Take a small commercial job. The assumptions, which you should replace with your own figures:
- 8 IP cameras, each main stream averaging 4 Mbps (a measured figure, not the setting).
- Continuous recording all day and night, so 86,400 seconds a day.
- Retention of 30 days, agreed with the owner.
Per day: 4 x 86,400 x 8 / 8 = 345,600 MB, or 345.6 GB. Over 30 days that is 10,368 GB, about 10.4 TB. Add roughly 20 percent headroom and the target is about 12.4 TB of usable space. The table repeats the sum for other average bitrates.
| Average bitrate | Per camera per day | Per camera, 30 days | 8 cameras, 30 days |
|---|---|---|---|
| 1 Mbps | 10.8 GB | 324 GB | 2.6 TB |
| 2 Mbps | 21.6 GB | 648 GB | 5.2 TB |
| 4 Mbps | 43.2 GB | 1.3 TB | 10.4 TB |
| 8 Mbps | 86.4 GB | 2.6 TB | 20.7 TB |
The table is arithmetic, not a forecast. Which row a camera lands in depends on its resolution, codec, frame rate and scene, so read the bitrate from the recorder before you pick a row.
Why the real bitrate differs from the setting
Most IP cameras default to variable bitrate (VBR). The number in the menu is a target or a cap, and the encoder spends more bits on a busy or noisy picture and fewer on a still one. Two cameras with identical settings can record very different amounts of data.
Common reasons the measured bitrate runs higher than expected:
- Night infrared: low-light noise and grain look like motion to the encoder.
- Busy scenes: traffic, trees in wind, rain and reflections.
- Wide views: more detail across the frame means more data.
The U.S. Department of Homeland Security’s Video Security Systems Technology Handbook lists resolution, image rate, motion detection features, complexity of image and field of view as the factors that set a recorder’s storage. The practical fix is to let the system run for a day and night, then read the real average per camera from the recorder’s stream statistics.
Continuous, motion and scheduled recording
Recording mode is the second big lever. The DHS handbook notes that some systems record only when activity is detected, while others record continuously at a low image rate and switch to a higher rate when activity starts. It also suggests higher rates for primary cameras and lower rates for secondary ones.
How much motion recording saves varies with the scene. A stockroom that sees two visits a day saves a great deal; a lobby or a road view may barely differ from continuous.
For a rough plan, estimate the share of the day each camera will actually record. If a camera records a third of the day, its storage is a third of the continuous figure: in the example, about 3.5 TB instead of 10.4 TB. Size the drives for the busiest week, not an average one.
Codec, resolution and frame rate
Compression decides how many bits each frame costs. The DHS handbook states that H.265 requires half the bandwidth of other standards. In practice the saving varies by camera, scene and settings, so treat any H.265 versus H.264 percentage as a starting estimate and confirm it with measured bitrates.
Resolution and frame rate push the other way. More pixels and more frames per second mean more data, and the handbook notes frame rate has a significant impact on the amount generated. Many installers record the main stream at a modest frame rate for most cameras and keep full rate for doors, registers and gates.
From terabytes on the label to days on the screen
Drive makers count a terabyte as 1,000,000,000,000 bytes. Many recorders and computers report capacity in binary units, so a drive labelled 4 TB shows as roughly 3.6 TB. No space has gone missing, but mixing the two units in one sum skews the retention estimate, so keep every figure in decimal units, as this guide does, and allow a little for the file system.
The handbook makes the stakes clear: the hard drive capacity determines how many days of recording remain before the system begins recording over the oldest video. Leave around 20 percent headroom for busier periods and added cameras.
NVR hard drive size, bays and redundancy
Once you have the usable target, turn it into drives. Check how many bays the recorder has and the largest drive it supports, then split the capacity across them. A recorder with several bays reaches a given total with smaller drives; a single-bay unit may cap retention before the camera count does.
- Drive type: use drives rated for continuous surveillance writing, not desktop drives.
- Redundancy: the handbook notes RAID spreads video across several drives to protect against a single point of failure. Mirroring halves usable space, and parity RAID costs one drive or more, so add that to the sum.
- Growth: spare bays let the owner extend retention later without a new recorder.
The recorder type sets where encoding happens; our DVR vs NVR guide covers that choice. Switch power for the cameras is in the PoE budget for IP cameras, and the cable in Cat5e vs Cat6 for security cameras.
Where to find recorders for the job
The Bursma recorders category lists DVRs and network recorders with drives fitted, including the CSIN16NS4TB 16 channel network recorder with 4TB and the CSIN8NSP2TB 8 channel recorder with built-in PoE and 2TB. Cameras are in the security cameras category, and the full range is in the security, surveillance and control division.
For pricing and availability, ask your sales representative through the Bursma contact page. If you install for a living, apply for a Bursma dealer account.
Frequently asked questions
How many days will a 4TB drive record?
Divide capacity by daily use. A 4 TB drive holds 4,000 GB; at 4 Mbps continuous, one camera uses 43.2 GB a day, so four cameras fill it in about 23 days, a little less after the file system takes its share. Your measured bitrate changes the answer.
Does H.265 use less storage than H.264?
Usually, at similar image quality. The DHS handbook says H.265 needs half the bandwidth of other standards, but the real saving varies by camera and scene, so measure it.
Should I size storage for motion recording or continuous?
Size for the mode the owner will actually run, and for the busiest period. If retention matters for evidence, many installers size for continuous recording and treat motion savings as extra headroom.
Can I use a desktop hard drive in an NVR?
It may work, but surveillance-rated drives are designed for continuous writing from many streams. Check the recorder’s supported drive list.